A nonthermal atmospheric pressure plasma source based on a dc-driven intermittent spark discharge in a coaxial electrode configuration with volumetrically distributed discharge filaments is presented. Spreading the intermittent spark discharge over the volume of the cylindrical discharge chamber is achieved owing to the mutual action of acoustic and magnetic fields on the discharge. The magnetic field scans discharge filaments over the cross section of the cylindrical electrode, while the acoustic field spreads the discharge along the electrode. Electrical parameters and characteristics of the developed nonthermal atmospheric pressure plasma source with volumetrically distributed plasma microchannels are presented.
Acoustic Waves Effect on the Generation of Nitrogen Oxides by Corona Discharge in Air
We studied the interaction of acoustic waves with negative corona discharge in air at atmospheric pressure. The application of acoustic waves on the discharge causes a mixing of ambient air with the gas between the electrodes, which results in cooling of the gas in the inter-electrode gap. As a rough indirect indicator of the discharge cooling, we used a concentration of nitrogen oxides generated by the discharge. We chose these oxides because they exhibit antibacterial properties. We showed that for a particular discharge power with increasing amplitudes of acoustic displacements, the concentration of generated nitrogen mono/dioxide decreases, and that this decrease could be attributed mainly to the temperature. We also found that application of acoustic waves on the discharge for a particular power substantially decreases transient spark frequency, increases the magnitude of the streamer onset voltage, and increases the difference between the streamer and spark onset voltage.
Acoustic field effects on a negative corona discharge
For a negative corona discharge under atmospheric pressure in different regimes, we investigated the effects of an acoustic field both on its electrical parameters and on the change in its visual appearance. We found that the application of an acoustic field on the true corona discharge, for particular currents, decreases the discharge voltage. The application of an acoustic field on the discharge in the filamentary streamer regime substantially extends the range of currents for which the discharge voltage remains more or less constant, i.e. it allows a substantial increase in the power delivered to the discharge. The application of an acoustic field on the discharge causes the discharge to spread within the discharge chamber and consequently, a highly reactive non-equilibrium plasma is created throughout the inter-electrode space. Finally, our experimental apparatus radiates almost no acoustic energy from the discharge chamber.
Acoustic plane waves in a gas-filled duct with an axial temperature gradient
Description and analysis of acoustic waves in ducts with a temperature gradient represents a signifi-
cant problem of science and practical interest. This problem is induced by the need of understanding
how temperature fields affect acoustic processes which leads to a better possibility to design and con-
trol systems in which interactions between the acoustic and temperature fields occur. This includes for
instance thermo-acoustic devices and engines, combustors, automotive mufflers, measuring methods
of impedances of high temperature systems, investigation of thermo-acoustic and combustion instabil-
ities etc. Most of the works dealing with these problems requires either the weak temperature gradient
or temperature gradients, which are not fully consistent with the imposed physical conditions. In our
work we deal with investigating of the behavior of plane acoustic waves that enter a region with
arbitrarily spatially varying temperature gradient. This temperature region smoothly verges into the
remaining regions in which the temperature gradient is constant. The one-dimensional wave equa-
tion for ducts with an axial mean temperature gradient is solved analytically. The derived solutions
extend the class of published analytical solutions of model wave equations taking into account the
temperature gradient of medium by solutions that assume a smooth temperature distribution.
Acoustically Stabilized Edge to Wire Electrode Electrical Discharges
We describe acoustical stabilization of the negative corona discharge in air with razor blade electrode against grounded wire electrode placed axially in a cylindrical resonator in the pressure node of the acoustic standing waves. When the standing waves are applied the discharge becomes more uniform and its volume is substantially increased along entire edge of the blade electrode. Images of the discharges for various acoustic field strengths are presented.
An acoustical resonator for stabilization of electrical discharge
This work deals with study of a device for an efficient generation of standing acoustic field with
high amplitude of acoustic particle displacement. The device is intended to be used for acoustic sta-
bilization of electrical discharges for enhancement of plasma-chemical reactions in ecological applica-
tions. It consists of two out-of-phase-driven loudspeakers interconnected with a symmetrically shaped
waveguide forming together a low-frequency electro-mechano-acoustical resonant system. A theoret-
ical model based on lumped-element circuits is proposed for description of the device, its tuning
capabilities and optimization. An analysis of the model is performed resulting in an experimental
sample realization. Acoustic measurements based on two-microphone method are conducted to prove
functionality of the device even in case of high-amplitude acoustic fields.
Interaction between two nonlinear acoustic resonators
This work is dedicated to problems connected with an interaction between two nonlinear acoustic cylindrical
resonators. The resonators are closed and interact due to an elastically mounted wall which is placed between
them. This wall represents a one-degree-of-freedom mechanical oscillator that is described by the linear
equation of motion. Acoustic fields inside the resonators are generated by vibrating pistons which are located
at their ends. The pistons are capable to excite nonlinear standing waves. For description of the waves
model equations were derived, which represent the modified inhomogeneous Burgers equations. Nonlinear
acoustic fields are coupled linearly by the elastically mounted wall. This resonant system enables to set
a lot of configurations which is due to many optional parameters, e.g. frequency and amplitude of the
vibrating pistons, detuning, characteristic frequency of the mechanical oscillator and its damping. This paper
contains interesting analysis of chosen configurations and demonstrates possibility of using of the coupled
inhomogeneous Burgers equations for these purposes.
The Acoustic Resonator Combined with Electrical Discharges
The important factors in industrial applications are discharge power and volume. In order to increase those and to prevent a discharge transition into sparks, we designed a discharge stabilization of multi-needle to axial-wire electrode system, which is situated in the pressure node of the acoustic resonator. The discharge streamers are strongly spread. The discharges are stabilized at all needles even if they are connected to the same potential. The needle tips are effectively cooled by the acoustic wave. Several multi-needle electrodes can be placed around the resonator diameter centered at the node plane so that the considerable resonator node volume is filled with the discharges. The stabilization may be applied to different regimes of discharges; e.g. corona, glow and streamer.
Ultrasonic Measurement of Skin Surface Deformation
This paper describes a development of a system for ultrasonic measurement of a human skin surface deformation in-vivo. A measuring method is based on pulse-echo measurements of an ultrasonic wave propagating in the skin surface. The ultrasonic wave is transmitted and received using a pair of piezoceramic transducers that are fifixed on the skin surface. The deformation of the skin surface is evaluated from changes of a distance between the ultrasonic
transducers. The developed measuring system is microprocessor controlled and consists of self designed transmitting and receiving circuits for evaluation of the distance between ultrasonic transducers. Measured data are transfered via USB interface. The system has been tested and considered as the working prototype. Further development will be focused on improvement of the measuring method and technique of the joint flexure range evaluation.
Air-Jet Power Ultrasonic Field Applied to Electrical Discharge
We describe a new setup of the Hartmann air-jet ultrasonic generator combined with electrical discharge in the nozzle-resonator gap. Using the schlieren visualization of air jet and ultrasonic field we investigated the shape and structure of the discharge and we determined relationship among the acoustic field in the nozzle-resonator gap, generator ultrasonic emission and discharge behavior.
High-Amplitude Standing Waves Between Collateral Discs
The paper is concerned with study of behaviour of high-amplitude standing acoustic waves
between two collateral discs whose dimensions are comparable with the wavelength. The
work is motivated by research of possibilities of acousto-optical imaging in gases. Firstly, the
system was numerically modeled in linear approximation using the Finite Elements Method in
order to assess its Q-factor and resonant frequencies for different discs' radius-wavelength
ratios. An approximate formula was found for calculation of resonance frequency for given
geometry. Secondly, high-amplitude waveforms and generation of higher harmonics was
studied using time-domain numerical integration of Navier-Stokes equations. It was observed
that typical shock-wave does not develop in spite of considerable amplitudes of acoustic
pressure that is caused by irregular distribution of resonant frequencies for individual modes.
Stabilization of Multi-needle-to-Plate Electrical Discharges in a High-intensity Sound Field
The efficiency of different chemical reactions can be improved by a power sound field in an acoustic resonator and by ionization of the reactant medium by electrical discharges. The important factors in the industrial applications are discharge power and volume. To increase the discharge volume and to prevent a discharge transition into sparks, we designed a new stabilization of multi-needle-to-plate electrode system. Stabilization factors are: the gas particles that are swung on the pressure node plane; the sound pressure gradient, which strongly influences breakdown conditions. With the abatement of sound pressure the discharge is strongly spread and stabilized at all needles even if they are connected to the same potential. The needle tips are effectively cooled by the sound wave. The stabilization may be applied to different regimes of discharges.
Hartmann Ultrasound Generator Combined with Electrical Discharge
We describe a new setup of the Hartmann gas-jet ultrasonic generator combined with the electrical discharge with reduced gas flow rate. Our idea involves a rod along the nozzle-resonator axis. New nozzle construction also enables to control electric field in the nozzle-resonator gap and creates high power ultrasoud field in volume in front of the resonator. Apart of the fact that the discharge is stabilized along the resonator orifice circumference, it becomes more uniform and it increases its volume.
High-amplitude acoustic field in a disc-shaped resonator
The work is concerned with study of high-amplitude acoustic fields in thin cylindrical discs, where the transversal mode is driven using a vibrating piston. Due to the fact that higher eigenfrequencies are not integer multiples of the eigenfrequency fundamental, excitation of shock-wave is avoided and nonlinear dissipation is supressed. The problem is described using a set of modified Navier-Stokes equations that are integrated numerically.
Ultrasonic Field Effects on Corona Discharge in Air
The application of ultrasonic waves on corona discharge causes existence of pressure gradients in the discharge gap. According to Meek´s criterion formation of streamers in the discharge and therefore the discharge profile is affected by these pressure gradients. We calculated the distribution of ultrasonic pressure in the interelectrode gap and we demonstrated experimentally that the discharge profile is affected by this ultrasonically induced pressure distribution.
Numerically simulated ultrasonic density and pressure fields are compared with the schlieren pictures of the same field. The results are used to explain the interaction of the gas flow with the ultrasonic field generated by the piston. Knowledge of these density distributions is required for the description of more complex processes, which take place if an electrical discharge is formed between the nozzle and the ultrasonic transducer. Such combined electrical and aerodynamic phenomena are presented in environmental applications such as production of ozone, VOC decomposition and de-NOx processes enhancement.
Schlieren Visualization of Ultrasound Wave against the Gas Flow through the Nozzle
The schlieren visualization of ultrasonic standing waves interacting with the propane-butane gas flow through the nozzle (hollow-needle) is described. The interaction takes place in one-wavelength ultrasonic resonator excited by a solid piston with only a slightly smaller diameter than the resonator cross-section. The attempt to explain the observed effects like disturbing the gas layer along the vibrating surface as well as efficient mixing of gas is presented.
Ultrasonic Resonator with Electrical Discharge Cell for Ozone Generation
The effect of ultrasound waves in an ultrasonic resonator cell on ozone generation by a hollow needle to plate electrical discharge enhanced by the airflow through the needle is experimentally investigated. It was found that the application of ultrasound waves increases ozone generation for discharge when the needle is negatively biased, and has no effect on ozone generation for the discharge when the needle is biased positively. We try to explain the influence of ultrasound on the discharge mechanisms.
The effect of power ultrasound application on DC hollow needle to plate atmospheric pressure electrical discharge enhanced by the flow of air through the needle electrode was studied experimentally. It was found that applying ultrasound increases discharge volume. In this volume take place plasmachemical processes, used in important ecological applications such as the production of ozone, VOC decomposition and de-NOx processes enhancement.
The Effect of Ultrasound on Hollow-Needle to Plate Electrical Discharge
The effect of ultrasound standing waves at a frequency of 20.3 kHz on hollow-needle to plate electrical discharge enhanced by the flow of air through the needle is described. It was found that the suitable application of ultrasound increases discharge volume in cylindrical one wavelength ultrasonic resonator excited by solid piston with only a slightly smaller diameter than the resonator diameter. This discharge-spread is connected with radial pressure distribution as a result of ultrasound interaction with the airflow from the needle. Efficient mixing in infinitesimal layer of the air created along vibrating electrode and other complex processes, which are caused by electrical and aerodynamic interactions can be used for environmental applications such as production of ozone, VOC decomposition and de-NOx processes enhancement.
The Measurement of High Power Ultrasonic Field in Cylindrical Resonator
This paper deals with the analysis of ultrasonic waves inside cylindrical resonator. These waves behave nonlinearly, so it is not easy to describe theoretically their behaviour. Therefore, we started with an experimental determination of acoustic pressure. We chose two methods of measurement - by a microphone and by an optical interferometric probe. A harmonic power ultrasonic wave at a base frequency of 20.3 kHz was generated by a piston. The glass cylindrical resonator with an internal diameter of 17 mm and adjustable reflector was used. The dependence of the harmonic components of the acoustic pressure on the distance from the source of the power ultrasound was measured. The data measured by the microphone and by the interferometer were compared.
Ultrasound and Airflow Induced Thermal Instability Suppression of DC Corona Discharge: an Experimental Study
We are particularly interested in the measurement of nonlinear acoustic waves in air, within the range of the Raman-Nath diffraction regime. We used a heterodyne laser interferometric system with a Bragg cell frequency of 80 MHz. A harmonic power acoustic wave with a frequency in the range of 20 kHz was generated, but higher harmonics appear when nonlinear propagation occurs in restricted volumes or waveguides.
Výzkum na katedře fyziky elektrotechnické fakulty ČVUT
Vizualizace poruch či nehomogenit v transparentním mediu je potřebná v mnoha situacích. Jednou z možností je šlírová metoda zobrazování. Tato technika umožňuje vidět neviditelné, v tomto příspěvku je popsaná interakce ultrazvukového pole s plynem proudícím z konce jehly. Byly popsány optické a elektronické součásti aparatury, která má vysoké rozlišení. Výsledku jsou prezentovány na fotografiích prostoru mezi koncentrátorem ultrazvukového generátoru a koncem jehly, pro zvyšující se tok plynu jehlou a pro případ zapnutého a vypnutého generátoru.
A comparison measurement of nonlinear ultrasonic waves in tubes by a microphone and by an optical interferometric probe
This paper deals with the analysis of ultrasonic fields inside waveguides generated by ultrasonic waves of high amplitude. These waves behave nonlinearly, so it is not possible to use standard linear equations to describe their behaviour. Therefore, we started with an experimental determination of the acoustic pressure of air in glass tubes. We chose two methods of measurement-by a microphone and by an optical interferometric probe. The conventional method by a microphone creates numerous problems, which can be avoided by using an optical method, a heterodyne laser interferometer.
Měření velkých akustických tlaků a výchylek laserovým interferometrem
Typical sources of non-equilibrium plasmas are barrier discharge, direct current (DC) or alternating current (AC) gliding arc, pulsed or DC corona and DC atmospheric pressure discharge stabilized by a fast gas flow (APD-GFS). In case of APD-GFS the gas flows in a rectangular channel, the top wall of which serves as the anode and the multi-needle cathode is built into the bottom wall of the channel. In order to prevent the transition to a spark and to stabilize this type of discharge the velocity of the gas should be about 100-200 m s-1 or the discharge current must be limited. To avoid the problem connected with the acceleration of the primary (polluted) gas at such a velocity, the external flow of the primary gas around the needle electrodes can be superimposed by a flow of a secondary gas through the needles. Thus the primary gas need not be accelerated to high velocity and in order to stabilize the discharge a relatively small amount of a secondary gas supplied through the needle is required. This work is therefore focused on the study of the DC APD-GFS in hollow needle-to-plane geometry. The basic electrical characteristics, magnetic noise and integral emission spectra of this type discharge with the flow of nitrogen or air through the needle are given.
Internally Cooled Needle to Plate Electrical Discharge
Proceedings of the 9th Regional Conference IUAPPA - 3rd International Conference, Environmental Impact Assessment, Vol.2. Praha: České vysoké učení technické v Praze, 1996, pp. 311-319. ISBN 80-01-01514-9.
Proceedings of the 9th Regional Conference IUAPPA - 3rd International Conference, Environmental Impact Assessment, Vol.2. Praha: České vysoké učení technické v Praze, 1996, pp. 326-334. ISBN 80-01-01514-9.
Proceedings of the 9th Regional Conference IUAPPA - 3rd International Conference, Environmental Impact Assessment, Vol.2. Praha: České vysoké učení technické v Praze, 1996, pp. 339-344. ISBN 80-01-01514-9.
Proceedings of the 9th Regional Conference IUAPPA - 3rd International Conference, Environmental Impact Assessment, Vol.2. Praha: České vysoké učení technické v Praze, 1996, pp. 351-358. ISBN 80-01-01514-9.
Proceedings of the 9th Regional Conference IUAPPA - 3rd International Conference, Environmental Impact Assessment, Vol.3. Praha: České vysoké učení technické v Praze, 1996, pp. 597-599. ISBN 80-01-01515-7.